Existing microwave antennas include a wide variety of configurations for various applications, such as satellite reception, remote broadcasting, or military communication. The desirable characteristics of low cost, light weight, low profile and mass producibility are provided in general by printed circuit antennas.
The simplest forms of printed circuit antennas are microstrip antennas wherein flat conductive elements, such as monopole or dipole antenna elements, are spaced from a single essentially continuous ground plane by a dielectric sheet of uniform thickness. An example of a microstrip antenna is disclosed in U.S. Pat. No. 6,417,813 to Durham, which is assigned to the current assignee of the present invention and is incorporated herein by reference in its entirety.
The antennas are designed in an array and may be used for communication systems requiring such characteristics as low cost, light weight and a low profile. The bandwidth of such antennas is about 10-to-1. However, a 10-to-1 bandwidth can be limiting for certain applications. For example, electronic warfare support measures (ESM) and electronic intelligence (ELINT) radar systems require antennas having a bandwidth typically greater than 20-to-1, which offers a higher probability of intercepting signals.
One approach for increasing the bandwidth of an array of dipole antenna elements is disclosed in U.S. Pat. No. 6,552,687 to Rawnick et al., which is also assigned to the current assignee of the present invention and is incorporated herein by reference in its entirety. The multiband phased array antenna in the '687 patent includes a first array of dipole antenna elements operating over a first frequency band, and a second array of dipole antenna elements operating over a second frequency band so that the phased array antenna is a multiband antenna.
The size of the dipole antenna elements in the first array is different from the size of the dipole antenna elements in the second array. Consequently, the ground plane spacing is different between the first and second arrays. One disadvantage of this configuration is that since the higher frequency dipole antenna elements are surrounded by the lower frequency dipole antenna elements, there is a gap or hole in the aperture distribution of the lower frequency dipole antenna elements. Consequently, the layout of the different size antenna elements in the '687 patent presents difficulties in controlling the antenna pattern since this gap or hole may have undesired effects, such as raising the sidelobe levels of the antenna. In addition, the fact that the physical aperture size does not change over a large bandwidth (approximately 10:1) means that the electrical size of the aperture will vary considerably over the band, making this approach unsuitable as a feed for a reflector.
A different type antenna that offers a wide bandwidth (greater than 20-to-1) is a spiral antenna. To cover multiple frequency bands, multiple spirals may be used, i.e., a spiral for each frequency band. However, the multiple spirals are non-concentric about the focal point of the antenna when operating as a feed for a reflector, which results in a loss of efficiency due to scan loss compared to that of a completely concentric aperture. In addition, another disadvantage is that the efficiency of spiral antennas is typically much less than 50% since their performance depends on an absorber-filled back cavity.
Consequently, the use of electromagnetically coupled dipole antenna elements is preferred over spiral antennas. However, utilizing an array of dipole antenna elements presents a dilemma. The maximum grating lobe free scan angle can be increased if the dipole antenna elements are spaced closer together, but a closer spacing can increase undesirable coupling between the elements, thereby degrading performance. This undesirable coupling changes rapidly as the frequency varies, making it difficult to maintain a wide bandwidth.
One approach for compensating the undesirable coupling between dipole antenna elements is disclosed in U.S. patent application Ser. No. 10/634,036 which is assigned to the current assignee of the present invention, and which is incorporated herein by reference. In particular, a respective impedance element is electrically connected between the spaced apart end portions of adjacent legs of adjacent dipole antenna elements for providing increased capacitive coupling therebetween.
The respective impedance elements may have different impedance values so that the frequency band of the phased array antenna can be tuned or adjusted for particular applications. Adjusting the frequency band typically includes increasing the lower or upper ranges thereof. Unfortunately, once an impedance element has been electrically connected between the spaced apart end portions of adjacent legs of adjacent dipole antenna elements, the phased array antenna cannot be retuned.